The Nativity Fast, practiced by the Eastern Orthodox Church and other churches such as the Eastern Catholic churches, is believed to enable participants to draw closer to God by denying the body of worldly pleasure in preparation for celebration of the birth (or Nativity) of Jesus, which is held on December 25 (Julian Calendar). Although the fast influences the body, the emphasis is placed on the spiritual facet of the fast rather than physical deprivation. Sometimes the fast is called the Philippian fast, as it traditionally follows the Feast of St. Philip the Apostle on November 14.

In the Eastern Orthodox Church, the fast goes from November 15 to December 24 and is traditionally from red meat, poultry, meat products, eggs, dairy products, fish, oil, and wine; where fish is allowed on Saturdays and Sundays, and oil and wine are allowed on Tuesdays and Thursdays. There has been some ambiguity about the restriction of fish, whether it means the allowance of invertebrate fish or all fish. More detailed guidelines vary, but the rules strictly state that from the 20th of December to the 24th of December, no fish may be eaten. On December 25, the Afterfeast of the Nativity of Christ to Theophany Eve begins, which declares the time period from December 25 to January 4 fast-free.

Some churches have abbreviated the fast to start on December 10, following the Feast of the Conception of Saint Anne.